[MemorySSA] Update Phi insertion.

Summary:
MemoryPhis may be needed following a Def insertion inthe IDF of all the
new accesses added (phis + potentially a def). Ensure this also  occurs when
only the new MemoryPhis are the defining accesses.

Note: The need for computing IDF here is because of new Phis added with
edges incoming from unreachable code, Phis that had previously been
simplified. The preferred solution is to not reintroduce such Phis.
This patch is the needed fix while working on the preferred solution.

Reviewers: george.burgess.iv

Subscribers: Prazek, sanjoy.google, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D67927

llvm-svn: 372673